[wpigui] Add wpigui wrappers for GLFW+imgui

These hide the platform specifics behind a common C++ API.  Platforms:
 - Windows: DirectX 11 (with 10 backwards compatibility)
 - Linux: OpenGL 3
 - Mac: Metal

wpigui/src/dev/native/cpp/main.cpp

@@ -0,0 +1,14 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2020 FIRST. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "wpigui.h"
+
+int main() {
+  wpi::gui::CreateContext();
+  wpi::gui::Initialize("Hello World", 1024, 768);
+  wpi::gui::Main();
+}